What is BMI?
Body Mass Index (BMI) is a simple number that compares your weight to your height. It is calculated as your weight in kilograms divided by the square of your height in metres. Doctors and health organisations use it as a quick screening tool to group adults into underweight, normal weight, overweight, and obese ranges defined by the World Health Organization.
This calculator works in both metric and imperial units and shows your result on a colour gauge so you can see exactly where you sit. BMI is a general guide for adults and does not account for muscle mass, body composition, age, or pregnancy, so very muscular people may read as overweight even when they are healthy. Use it as one signal among many.
BMI categories (WHO)
Below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. These ranges apply to most adults aged 20 and over.
Frequently asked questions
How is BMI calculated?
BMI equals weight in kilograms divided by height in metres squared. In imperial units it is weight in pounds times 703 divided by height in inches squared.
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is considered the normal, healthy range according to the World Health Organization.
Is BMI accurate for everyone?
It is a general screening tool. It does not separate muscle from fat, so athletes may read high, and it is not used the same way for children, pregnant people, or the elderly.
Does BMI differ for men and women?
The standard adult BMI categories are the same for men and women, though body composition naturally differs. Some clinicians also look at waist measurement alongside BMI.
Is my data sent anywhere?
No. The calculation happens entirely in your browser. Nothing you enter is uploaded or stored.